Finally, physics’s zoo of subatomic particles is full. Scientists have almost certainly snared the Higgs boson, the last particle waiting to be roped into the fold.

Decades after it was proposed, the Higgs emerged in the shards of particle collisions at the world’s most powerful accelerator, the Large Hadron Collider at the CERN laboratory near Geneva. Physicists announced the discovery on July 4 during a seminar at the lab.

“We have now found the last missing cornerstone of the standard model,...